Reference chart
Data Size and Speed Chart
By &Pixels, the studio that builds MyCalculator.to. Decimal and binary units are given side by side, because both are in everyday use and they are not the same.
Why a 1 TB drive shows 931 GB, why a 100 Mbps connection downloads at 12.5 MB/s, and the two factors that explain almost every confusing number in computing.
What is the difference between GB and GiB?
A gigabyte is 1,000,000,000 bytes. A gibibyte is 1,073,741,824, which is two to the thirtieth power. Storage and networking use the decimal units; operating systems and memory largely use the binary ones and then label them with the decimal names.
That single inconsistency explains most of the confusing numbers in computing, and the gap grows with scale: 2.4 percent at kilobytes, 7.4 percent at gigabytes, 10 percent at terabytes. It is why a new drive always looks smaller than the box said.
Decimal against binary
| Unit | Decimal | Binary | Gap |
|---|---|---|---|
| Kilobyte (KB) | 1,000 bytes | 1,024 bytes (KiB) | 2.4% |
| Megabyte (MB) | 1,000 KB | 1,048,576 bytes (MiB) | 4.9% |
| Gigabyte (GB) | 1,000 MB | 1,073,741,824 bytes (GiB) | 7.4% |
| Terabyte (TB) | 1,000 GB | 1,099,511,627,776 bytes (TiB) | 10.0% |
| Petabyte (PB) | 1,000 TB | 1,125,899,906,842,624 bytes (PiB) | 12.6% |
The calculators behind this
What a drive actually shows
| Sold as | Reported as | Apparent shortfall |
|---|---|---|
| 256 GB | 238 GiB | 18 GB |
| 500 GB | 465 GiB | 35 GB |
| 1 TB | 931 GiB | 69 GB |
| 2 TB | 1.82 TiB | 180 GB |
| 4 TB | 3.64 TiB | 360 GB |
| 8 TB | 7.28 TiB | 720 GB |
Connection speed to download speed
The other factor of eight. Connections are sold in megabits and files are measured in megabytes, so the advertised number is always eight times the one you see downloading. The last column is a 1 GB file.
| Plan | Download rate | 1 GB file |
|---|---|---|
| 25 Mbps | 3.1 MB/s | about 5 min 20 s |
| 50 Mbps | 6.3 MB/s | about 2 min 40 s |
| 100 Mbps | 12.5 MB/s | about 1 min 20 s |
| 300 Mbps | 37.5 MB/s | about 27 s |
| 500 Mbps | 62.5 MB/s | about 16 s |
| 1 Gbps | 125 MB/s | about 8 s |
How to read any of these numbers
- Check whether it is bits or bytes. A lowercase b is bits, an uppercase B is bytes. The difference is a factor of eight.
- Divide a connection speed by 8. A 100 Mbps line moves about 12.5 MB per second at best, before any overhead.
- Expect a drive to show less than the box. The box counts in thousands, the operating system counts in 1,024s. Nothing is missing.
- Use GiB when the distinction matters. Kibibyte, mebibyte and gibibyte exist precisely so that a number can be unambiguous.
- Take 10 to 20 percent off for real transfers. Protocol overhead, contention and the far end all take a share of any advertised rate.
For a specific conversion, the file size converter handles both systems and labels which one it used, and the download speed calculator takes a file size and a connection speed and gives a realistic time rather than the theoretical one.
Formula & Methodology
Formula
seconds = (file MB x 8) / connection Mbpsfile MB = file size in megabytes, as your computer reports it
8 = bits per byte, the factor the two units differ by
connection Mbps = the advertised line speed, in megabits
Worked Example
A 1 GB file on a 100 Mbps connection
Did you know? The kibibyte, mebibyte and gibibyte were standardised by the IEC in 1998 to end this ambiguity. Nearly thirty years on, storage still sells in decimal units and operating systems still report binary ones under decimal names, so the confusion the standard was written to fix is entirely intact.
Sources
- IEC 80000-13, quantities and units for information science and technology
- NIST guidance on binary prefixes for units of information